1. Field of the Invention
The present invention relates to optical disk drives and more particularly to an optical disk drive in which information is read/written by translating an optical pickup in a radial direction of an optical disk.
2. Description of the Prior Art
Optical disk drives for driving optical disks such as a compact disk (CD), a magneto-optical disk and a mini disk (MD) are provided with an optical pickup for illuminating an optical disk with a laser beam and receiving light reflected by the optical disk.
FIG. 1 shows a construction of an optical pickup mounted on a conventional optical disk drive. In an optical pickup 11 shown in FIG. 1, a laser beam 12.sub.a emitted by a semiconductor laser (light source) 12 passes through a beam splitter 14 via a coupling lens 13 and is incident on an optical disk 17 via a quarter-wave plate 15 and an objective lens 16.
Light reflected by the optical disk 17 travels through the objective lens 16 and the quarter-wave plate 15, and has its course changed by the beam splitter 14. The reflected light is caught by a first photosensitive element (photodiode) 19 via a convergent lens 18. A portion of the reflected light is caught by a second photosensitive element 23 via a reflecting mirror 20.
The objective lens 16 is provided with a coil 21 acting as an actuator that realizes tracking servo operation, and a coil 22 that realizes focusing servo operation.
FIG. 2 is a diagram showing a principal idea behind an operation for driving the optical pickup of FIG. 1. Referring to FIG. 2, the optical pickup 11 is engaged with and guided by guide shafts 23.sub.a and 23.sub.b provided to lie in a direction in which the optical pickup 11 is translated. A rack 24 is formed at an end of the optical pickup 11.
A driving gear 26 is fitted to a rotation shaft 25.sub.a of a motor 25 for driving the optical pickup 11. The driving gear 26 is engaged with a pinion 28 via gears 27.sub.a and 27.sub.b. The pinion 28 is engaged with the rack 24 of the optical pickup 11 so as to transmit a driving force. As a result, the optical pickup 11 is translated in a radial direction of the optical disk 17 along the guide shafts 23.sub.a and 23.sub.b.
A flexible printed circuit board 29 transmits a servo signal and the like.
In a write mode, the optical pickup 11 projects the high-power laser beam 12.sub.a from the semiconductor laser 12 on to the optical disk 17 embodied, for example, by a magneto-optic disk or an MD. Writing operation is effected by magnetizing a reflecting surface of the optical disk 17 with a magnetic head (not shown).
In a read mode, the optical disk 11 projects the low-power laser beam 12.sub.a on to the optical disk 17 (such as a compact disk). Reading is effected by demodulating reflected light caught by the first photosensitive element 19. An output signal of the first photosensitive element 19 is used in focusing servo operation and a restored data signal is derived from an output from the second photosensitive element 23.
While FIG. 1 shows an optical system for a main beam, the optical pickup 11 is also provided with two optical systems for a side beam. These additional optical systems are employed to generate an error signal for use in tracking servo operation.
FIG. 3 is a flowchart showing a conventional operation for causing the optical pickup 11 to jump across tracks. Referring to FIG. 3, when a host system issues a seek command for initiating a search, the motor 25 is driven so as to translate the optical pickup 11 to an initially set (S1) position. The optical pickup 11 then reads its current address (S2), and calculates the number of tracks to be traversed before reaching a target address based on the address read (S3).
A determination is then made as to whether or not the number of tracks to be traversed in a seek operation is zero (S4). If the number of tracks to be traversed is zero, that is, if it is found that the optical pickup 11 is already located on a target track, the search operation is terminated.
If the number of tracks to be traversed is not zero, a determination is made as to whether or not the actuator embodied by the coil 21 in the optical pickup 11 is capable of effecting a jump across the tracks determined to be traversed (S5). If the actuator is not able to effect a necessary jump, the number of steps required in a seek operation using the motor 25 and a thread mechanism (rack-and-pinion mechanism) is calculated (S6). A seek operation involving the calculated number of steps is executed and the control is returned to S2 (S7).
If it is found fin S5 that the actuator is capable of effecting a necessary jump across tracks, the coil 21 is fed a current so that a track jump using the actuator is carried out and the control is returned to S2 (S8).
Japanese Laid-Open Patent Application No. 3-16066 discloses a method for effecting a track jump. In the method disclosed in Japanese Laid-Open Patent Application No. 3-16066, a yoke of a feed motor (which corresponds to the motor 25 shown in FIG. 2) of an optical pickup is provided with not only a driving coil but also a Hall element for detecting rotation. Accordingly, a signal indicating a position is obtained by detecting rotation of a magnet provided in a rotor.
More specifically, a microcomputer provided in a disk drive calculates a position of a current track based on a linear velocity of the optical disk so as to determine the number of tracks to be traversed before a target address is reached. The number of steps to be executed in a translation is preset in a counter provided in the microcomputer. In accordance with FG pulses output from the Hall element in response to the rotation of the feed motor, the count of the counter is decremented. A velocity of the feed motor is controlled according to velocity data associated with the count so that the operation of the motor is stopped when the count reaches zero.
For example, assuming that 100 tracks exist in an interval between FG pulse edges and a track pitch is 1.6 .mu.m, position of the optical pickup is controlled with a 160 .mu.m resolution. Fine adjustment within the 160 .mu.m range is effected by a track jump using an actuator, based on a track count.
In this way, an accurate track jump can be effected without using a track count obtained by the optical pickup.
It is to be noted in a track jump method disclosed in Japanese Laid-Open Patent Application No. 3-16066 that the optical pickup is stopped when it is found that FG pulses has brought the count to zero. However, it is difficult to stop the optical pickup at a pulse edge. The optical pickup may stop at a position past the pulse edge.
FIG. 4 explains how the objective lens is moved in an access operation. FIGS. 5 and 6 explain problems with the conventional technology.
As shown in (A)-(C) of FIG. 4, the optical pickup 11 is translated either toward a center or a periphery of the optical disk so as to reach a target position. In its movement, the optical pickup 11 undergoes a violent acceleration and deceleration according to velocity data. Moreover, since tracking servo operation using an actuator is OFF during an access operation, the objective lens 16 undergoes a swinging motion in the access operation. Thus, in many cases, the objective lens 16 is displaced in an access direction when the optical pickup 11 comes to a halt. Specifically, the objective lens 16 may be displaced from a center of the optical pickup 11 by a distance corresponding to several FG pulses (200-300 .mu.m).
The optical pickup 11 reads an address of the displaced position, resulting in an erroneous identification of the position. The feed motor 25 is driven so that the optical pickup 11 is translated backward, producing another displacement of the objective lens 16 in an access direction. As shown in FIG. 5, this may result in the optical pickup 11 being moved endlessly by the motor 25.
Physically, the FG pulse edge does not always coincide with the track on the optical disk 17. There is a variation in the number of tracks existing between FG pulses, even more so when an eccentricity of the optical disk 17 exists. Thus, an error may exit between the driving of the pickup 11 by the motor 25 according to the pulse count and the actual movement of the pickup 11. Consequently, the seek operation may be repeated in four to six steps.
Further, the rotation shaft 25.sub.a may be twisted in a counter direction after the optical pickup 11 comes to a halt, due to a cogging (a backlash caused by the gears 27.sub.a and 27.sub.b, and the pinion 28) of the feed motor 25. As shown in FIG. 6, if the number of tracks existing between an address read in the position shown in FIG. 6 and a target address corresponds to one step of the FG pulse or more, the feed motor 25 is activated and stopped in response to a pulse edge. If another backward twist of the rotation shaft 25.sub.a occurs, the seek operation for one step is repeated.